Film forming method and plasma chemical vapor deposition apparatus

ABSTRACT

A PCVD apparatus includes a waveguide member which supports the workpiece with a portion of the waveguide member positioned in a reactor and causes microwaves output from a high-frequency output device to propagate to the workpiece. In a process of gradually increasing an intensity of the microwaves propagating to the workpiece through the waveguide member from “0”, the intensity of the microwaves output from the high-frequency output device when step-up of a bias current of the workpiece occurs is referred to as a first intensity, and in a process of gradually increasing the intensity of the microwaves from the first intensity, the intensity of the microwaves when step-up of the bias current occurs again is referred to as a second intensity. During film formation, the microwaves having an intensity of higher than the first intensity and lower than the second intensity are output from the high-frequency output device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Division of application Ser. No. 15/442,121 filed on Feb. 24,2017, which in turn is now allowed and claims priority to JP 2016-038873filed on Mar. 1, 2016. The disclosure of each of the prior applicationsis hereby incorporated by reference herein in its entirety.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2016-038873 filed onMar. 1, 2016 including the specification, drawings and abstract isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a film forming method of forming adiamond-like carbon film on a workpiece installed in a reactor, and aplasma chemical vapor deposition apparatus used in the film formingmethod.

2. Description of Related Art

An example of a plasma chemical vapor deposition apparatus (hereinafter,also referred to as “PCVD apparatus”) is described in Japanese PatentApplication Publication No. 2004-47207 (JP 2004-47207 A). The PCVDapparatus includes a reactor to which a process gas is supplied, awaveguide for supplying microwaves, and a dielectric window for closingan opening provided in a side wall of the reactor. In the reactor, thedielectric window supports a workpiece, and outside the reactor, the tipend of the waveguide is in contact with the dielectric window.

As microwaves propagate from the waveguide to the surface of thedielectric window, the process gas is decomposed into plasma in thevicinity of the dielectric window in the reactor. In addition, as thedecomposed process gas adheres to the workpiece, a film based on the gasis generated on the workpiece.

The formation of a diamond-like carbon film (hereinafter also referredto as “DLC film”) on the workpiece can be realized by supplying ahydrocarbon gas such as acetylene as the process gas into the reactor,and decomposing the hydrocarbon gas into a plasma.

However, the DLC film may be formed on a member which requires both highhardness and a low coefficient of friction, such as a plunger of ahigh-pressure fuel pump provided in a fuel supply system of an internalcombustion engine. In particular, it is desirable for the DLC filmformed on the plunger to achieve both high hardness and a lowcoefficient of friction to a high level.

SUMMARY OF THE INVENTION

The present invention provides a film forming method and a PCVDapparatus capable of forming a DLC film that achieves both high hardnessand a low coefficient of friction to a high level, on a workpiece.

A film forming method is a method of forming a diamond-like carbon filmon a workpiece installed in a reactor by decomposing a hydrocarbon gassupplied into the reactor of a plasma chemical vapor depositionapparatus (hereinafter, also referred to as “PCVD apparatus”) intoplasma using the PCVD apparatus. The plasma chemical vapor depositionapparatus includes a high-frequency output device which outputsmicrowaves, and a waveguide member which extends to an inside of thereactor from an outside of the reactor, supports the workpiece with aportion of the waveguide member positioned in the reactor, and causesthe microwaves output from the high-frequency output device to propagateto the workpiece. The microwaves having an intensity of higher than afirst intensity and lower than a second intensity are output from thehigh-frequency output device when a film is formed on the workpiecesupported by the waveguide member. In a process of gradually increasingthe intensity of the microwaves propagating to the workpiece through thewaveguide member from “0”, the intensity of the microwaves output fromthe high-frequency output device when step-up of a bias current of theworkpiece occurs is referred to as the first intensity, and in a processof gradually increasing the intensity of the microwaves from the firstintensity, the intensity of the microwaves when step-up of the biascurrent of the workpiece occurs again is referred to as the secondintensity.

In this configuration, since the waveguide member directly supports theworkpiece in the reactor, the microwaves supplied through the waveguidemember directly propagate to the workpiece. In this case, in thereactor, the hydrocarbon gas is decomposed into plasma around theworkpiece. Therefore, unlike a case where a dielectric is interposedbetween a waveguide and a workpiece, the diamond-like carbon film(hereinafter, referred to as “DLC film”) based on the hydrocarbon gasdecomposed into plasma can be formed on the workpiece even though theintensity of the microwaves output from the high-frequency output deviceis low. In other words, in a case where a film is formed on theworkpiece using the apparatus in which the dielectric is interposedbetween the waveguide and the workpiece, a DLC film cannot be formed onthe entire workpiece unless microwaves having a relatively highintensity are output from the high-frequency output device. Contrary tothis, in this configuration, the microwaves can directly propagate tothe workpiece from the waveguide member, and thus the DLC film can beformed on the entire workpiece even though the intensity of themicrowaves output from the high-frequency output device is relativelylow.

In addition, the DLC film is a film in which carbon having a diamondstructure and carbon having a carbon structure coexist with each other,and the hardness of the DLC film increases as the proportion of carbonhaving the diamond structure increases. In addition, since the DLC filmis formed on the workpiece using the hydrocarbon gas, hydrogen compoundsare contained in the DLC film as impurities. In addition, as the amountof hydrogen compounds increases, the hardness of the DLC film decreases.Therefore, in order to increase the hardness of the DLC film, it isdesirable to increase the proportion of carbon having the diamondstructure and to decrease the amount of hydrogen compounds.

In addition, in a case where the number of dangling bonds per unitvolume is referred to as a content, as the content of dangling bonds inthe DLC film increases, the amount of bonds between carbon atoms andhydroxy groups on the surface of the DLC film is likely to increase. Inaddition, as the amount of hydroxy groups bonded to carbon atoms on thesurface of the DLC film increases, the coefficient of friction of theDLC film can be decreased.

A hydrocarbon molecule has two or more carbon atoms and two or morehydrogen atoms. In this hydrocarbon molecule, the bonds between thecarbon atoms include π bonds and a σ bond, and the bond strength of theσ bond is higher than that of the π bond. In addition, the strength ofthe bond between the carbon atom and the hydrogen atom is higher thanthe strength of the π bond and is lower than the strength of the σ bond.

The inventors obtained the following knowledge regarding therelationship between the intensity of the microwaves and the hardnessand coefficient of friction of the DLC film in a case where the DLC filmis formed on the workpiece using the PCVD apparatus that directlysupports the workpiece with the waveguide member. That is, in a casewhere the bias current of the workpiece is observed when the intensityof the microwaves propagating to the workpiece through the waveguidemember is gradually increased from “0”, step-up of the bias currentoccurs when the intensity of the microwaves output from thehigh-frequency output device exceeds the first intensity, and step-up ofthe bias current occurs again when the intensity of the microwavesthereafter exceeds the second intensity.

In a case where the intensity of the microwaves output from thehigh-frequency output device during the film formation on the workpieceis equal to or lower than the first intensity, the energy when thehydrocarbon gas is decomposed into plasma is low, and most of the πbonds between the carbon atoms and the bonds between the carbon atomsand the hydrogen atoms are not broken and remain. In this case, a largeamount of π bonds between the carbon atoms remain in the hydrocarbon gasdecomposed into plasma, and thus the proportion of carbon having thediamond structure among the types of carbon contained in the DLC film islow. In addition, since not a significant amount of bonds between thecarbon atoms and the hydrogen atoms are broken, the content of danglingbonds in the DLC film generated on the workpiece is low, and the amountof hydrogen compounds in the DLC film increases. Therefore, the DLC filmformed under such conditions has low hardness and a high coefficient offriction.

In addition, in a case where the intensity of the microwaves output fromthe high-frequency output device during the film formation on theworkpiece is equal to or higher than the second intensity, the energywhen the hydrocarbon gas is decomposed into plasma is too high, and notonly the π bonds between the carbon atoms and the bonds between thecarbon atoms and the hydrogen atoms, but also the σ bond between thecarbon atoms are easily broken. As described above, since the bondsbetween the carbon atoms and the hydrogen atoms are easily broken, theDLC film in this case has a high content of dangling bonds and a lowamount of hydrogen compounds. In addition, atoms may be bonded togetheragain in a process of adhering to the workpiece in the hydrocarbon gasdecomposed into plasma as described above. However, in order for carbonatoms to be bonded together to form a σ bond, a higher energy than thatin a case where carbon atoms are bonded together to form π bonds isnecessary. That is, carbon atoms easily form π bonds but are less likelyto form σ bonds. Therefore, in the DLC film formed on the workpiece, thenumber of molecules with carbon atoms forming σ bonds is small, and theproportion of carbon having the diamond structure among the types ofcarbon contained in the DLC film is low. Therefore, in the DLC filmformed under such conditions, although the coefficient of friction islow, the hardness is not so high.

Contrary to this, in a case where the intensity of the microwaves outputfrom the high-frequency output device during the film formation on theworkpiece is higher than the first intensity and lower than the secondintensity, the energy when the hydrocarbon gas is decomposed into plasmais not too high, and thus the π bonds between the carbon atoms and thebonds between the carbon atoms and the hydrogen atoms are easily broken,while the σ bond between the carbon atoms is less likely to be broken.That is, since most of the bonds between the carbon atoms and thehydrogen atoms are broken, in the DLC film formed on the workpiece, thecontent of dangling bonds is high and the amount of hydrogen compoundsis small. In addition, since a large amount of σ the bonds betweencarbon atoms remain while the π bonds between the carbon atoms arebroken, the proportion of carbon having the diamond structure among thetypes of carbon contained in the DLC film is high. Therefore, in the DLCfilm formed under such conditions, the coefficient of friction is lowand the hardness is high. In addition, in the case where a film isformed on the workpiece by the apparatus in which the dielectric isinterposed between the waveguide and the workpiece rather than bycausing the microwaves to directly propagate to the workpiece from thewaveguide member, the intensity of the microwaves output from thehigh-frequency output device typically becomes higher than the secondintensity.

Therefore, in this configuration, by outputting the microwaves having anintensity of higher than the first intensity and lower than the secondintensity from the high-frequency output device, the microwaves can becaused to propagate to the workpiece installed in the reactor such thatthe DLC film is formed on the workpiece. Therefore, a DLC film whichachieves both high hardness and a low coefficient of friction to a highlevel can be formed on the workpiece.

In addition, in the film forming method, when the DLC film is formed onthe workpiece by causing the microwaves to propagate to the workpieceinstalled in the reactor, the workpiece may be charged with a negativecharge by supplying DC current to the workpiece. In this configuration,the microwaves propagate to the workpiece charged with the negativecharge. Therefore, compared to a case where the DLC film is formed onthe workpiece which is not charged with a negative charge, when the gasdecomposed into plasma is attracted to the workpiece, the gas is morelikely to evenly adhere to the entire workpiece.

For example, as the waveguide member of the PCVD apparatus, a waveguidemember which has an elongated first conductor of which one end ispositioned in the reactor to support the workpiece with the one end, anda cylindrical second conductor which is positioned on an outerperipheral side of the first conductor and is disposed coaxially withthe first conductor can be used. In a case of forming a DLC film on theworkpiece using this apparatus, the workpiece may be charged with anegative charge by supplying a DC voltage to the first conductor and themicrowaves output from the high-frequency output device may be caused toflow through the surface of the first conductor.

In this configuration, the first conductor for supplying the microwavesto the workpiece is also used for supplying DC current to the workpiece.Therefore, compared to a case where a supply path for DC current isprovided separately from a supply path for microwaves, film formation onthe workpiece can be performed with an apparatus having a simpleconfiguration. In addition, since the second conductor is disposed onthe outer peripheral side of the first conductor, leakage of themicrowaves to the outside of the apparatus can be prevented.

In the film forming method, the microwaves having an intensity of higherthan an intermediate value between the first intensity and the secondintensity and lower than the second intensity may be output from thehigh-frequency output device. Accordingly, a DLC film which achievesboth high hardness and a low coefficient of friction to a higher levelcan be formed on the workpiece W.

The PCVD apparatus is an apparatus which forms a diamond-like carbonfilm on a workpiece installed in a reactor by decomposing a hydrocarbongas supplied into the reactor into plasma. The PCVD apparatus includes:a high-frequency output device which outputs microwaves; and a waveguidemember which extends to an inside of the reactor from an outside of thereactor, supports the workpiece with a portion of the waveguide memberpositioned in the reactor, and causes microwaves output from thehigh-frequency output device to propagate to the workpiece. Thehigh-frequency output device outputs the microwaves having an intensityof higher than a first intensity and lower than a second intensity whena film is formed on the workpiece supported by the waveguide member. Ina process of gradually increasing an intensity of the microwavespropagating to the workpiece through the waveguide member from “0”, theintensity of the microwaves output from the high-frequency output devicewhen step-up of a bias current of the workpiece occurs is referred to asthe first intensity, and in a process of gradually increasing theintensity of the microwaves from the first intensity, the intensity ofthe microwaves when step-up of the bias current of the workpiece occursagain is referred to as the second intensity. In this configuration, thesame effects as those of the film forming method can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance ofexemplary embodiments of the invention will be described below withreference to the accompanying drawings, in which like numerals denotelike elements, and wherein:

FIG. 1 is a sectional view schematically illustrating a part of a plasmachemical vapor deposition apparatus used in a film forming method of anembodiment;

FIG. 2 is a schematic view showing the molecular structure of acetyleneused in the film forming method;

FIG. 3 is a graph showing a change in the bias current of a workpiecewhen the intensity of microwaves output from a high-frequency outputdevice of the plasma chemical vapor deposition apparatus is increasedfrom “0”;

FIG. 4 is a graph showing an example of the spectrum of plasma generatedin a reactor;

FIG. 5 is a graph showing a change in a C₂/C_(n)H_(m) emission intensityratio when the intensity of the microwaves output from thehigh-frequency output device of the plasma chemical vapor depositionapparatus is increased from “0”;

FIG. 6 is a graph showing the relationship between a first intensity, asecond intensity, and a pressure in the reactor;

FIG. 7 is a graph showing the relationship between the first intensity,the second intensity, and the concentration of the acetylene in a mixedgas supplied into the reactor;

FIG. 8 is a graph showing the relationship between the first intensity,the second intensity, and a DC voltage supplied from a DC power source;

FIG. 9 is a graph showing the relationship between the intensity of themicrowaves output from the high-frequency output device, the coefficientof friction of a diamond-like carbon film formed on the workpiece, andthe content of dangling bonds of the film; and

FIG. 10 is a graph showing the relationship between the intensity of themicrowaves output from the high-frequency output device, the hardness ofthe diamond-like carbon film formed on the workpiece, and the proportionof carbon having a diamond structure among the types of carbon containedin the film.

DETAILED DESCRIPTION OF EMBODIMENTS

An embodiment of a film forming method of forming a diamond-like carbonfilm on a workpiece and a plasma chemical vapor deposition apparatusused in the film forming method will be described with reference toFIGS. 1 to 10. In this specification, the diamond-like carbon film isreferred to as “DLC film”, and the plasma chemical vapor depositionapparatus is referred to as “PCVD apparatus”.

As shown in FIG. 1, a PCVD apparatus 11 includes a reactor 12 in which aworkpiece W made of a conductive material such as a metal is installed.In the reactor 12, a mixed gas containing acetylene, which is an exampleof a hydrocarbon gas, and an inert noble gas such as argon is suppliedto the vicinity of the installation position of the workpiece W by thesupply unit 13. The pressure in the reactor 12 is maintained at aspecified pressure by an operation of a vacuum pump 14.

In addition, in the PCVD apparatus 11, a waveguide member 20 forinputting microwaves to the workpiece W installed in the reactor 12 isprovided. The waveguide member 20 includes a cylindrical first conductor21 having an elongated shape, and a cylindrical second conductor 22which is positioned on the outer peripheral side of the first conductor21 and is disposed coaxially with the first conductor 21. A space isformed between an inner surface 221 of the second conductor 22 and anouter surface 211 of the first conductor 21. That is, the outside air(air) as an insulating layer, which flows in through a through-hole 212described later, is interposed between the first conductor 21 and thesecond conductor 22. In addition, a seal member 23 for restricting theinflow of the outside air into the reactor 12 is disposed between thesecond conductor 22 and the first conductor 21. The inner peripheralsurface of the seal member 23 is in close contact with the outer surface211 of the first conductor 21 and the outer peripheral surface of theseal member 23 is in close contact with the inner surface 221 of thesecond conductor 22. The seal member 23 is made of an insulatingmaterial that allows microwaves to pass therethrough.

The tip end of the first conductor 21 (that is, the upper end in thefigure) is positioned in the reactor 12, and the workpiece W isinstalled on the tip end. That is, the tip of the first conductor 21positioned in the reactor 12 serves as a support portion 21A thatdirectly supports the workpiece W.

The second conductor 22 is grounded, and thus the potential of thesecond conductor 22 is “0 V”. The tip end of the second conductor 22(that is, the upper end portion in the figure) enters the reactor 12through an opening portion 121 provided in the side wall of the reactor12.

Furthermore, the PCVD apparatus 11 includes a high-frequency outputdevice 30 which outputs microwaves and a DC power source 31 whichsupplies a DC voltage. The high-frequency output device 30 is providedwith an output unit 301 which outputs microwaves, and the output unit301 is connected to the first conductor 21 by passing through thethrough-hole 212 provided in the second conductor 22, that is, withoutcoming into contact with the second conductor 22. The microwaves outputfrom the high-frequency output device 30 flow through the outer surface211 which is the surface of the first conductor 21. At this time,leakage of the microwaves flowing through the outer surface 211 of thefirst conductor 21 to the outside of the apparatus is prevented by thesecond conductor 22.

In addition, the DC power source 31 is connected to the first conductor21, and the DC voltage is supplied from the DC power source 31 to thefirst conductor 21. DC current flowing through the first conductor 21also flows to the workpiece W supported by the first conductor 21.Accordingly, the workpiece W is charged with a negative charge. In thisembodiment, the magnitude of the DC voltage supplied from the DC powersource 31 is fixed.

During the formation of a film on the workpiece W, in a state in whichthe workpiece W is charged with the negative charge, microwaves areoutput from the high-frequency output device 30. Accordingly, themicrowaves propagate to the surface of the workpiece W which is chargedwith the negative charge, and acetylene is decomposed into plasma in thevicinity of the workpiece W in the reactor 12. As a result, a DLC filmbased on the acetylene is formed on the surface of the workpiece W.

Moreover, in the PCVD apparatus 11 of this embodiment, an ammeter 32 fordetecting a bias current of the workpiece W supported by the supportportion 21A in the reactor 12 is provided. The ammeter 32 is disposed onan electric wire that connects the first conductor 21 and the DC powersource 31 to each other. The “bias current of the workpiece W” is acurrent that flows into the DC power source 31 via the first conductor21 from the workpiece W when the microwaves are input to the workpiece Wand plasma is generated around the workpiece W.

Next, the molecular structure of the acetylene will be described withreference to FIG. 2. As illustrated in FIG. 2, acetylene has two carbonatoms and two hydrogen atoms. In addition, the bonds between the carbonatoms include π bonds, and a σ bond having a higher bond strength thanthe π bonds. In addition, in acetylene, the carbon atom and the hydrogenatom are bonded together. The strength of the bond between the carbonatom and the hydrogen atom is higher than the strength of the π bond andis lower than the strength of the σ bond.

When the DLC film is formed on the workpiece W, acetylene is decomposedand adheres to the workpiece W. In order to cause the coefficient offriction of the DLC film formed on the workpiece W as described above tobe low, in a case where the number of dangling bonds per unit volume isreferred to as a content, it is desirable that the content of danglingbonds in the DLC film is increased. This is because as the content ofdangling bonds increases, the amount of bonds between carbon atoms andhydroxy groups on the surface of the DLC film is likely to increase. Inorder to increase the content of dangling bonds in the DLC film, it isnecessary to break more bonds between carbon atoms and hydrogen atomswhen acetylene is decomposed into plasma.

In addition, when the DLC film formed on the workpiece W contains alarge amount of hydrogen compounds, the hardness of the DLC filmdecreases. The DLC film is a film in which carbon having a diamondstructure (also called “sp3 structure”) and carbon having a carbonstructure (also called “sp2 structure”) coexist with each other, and thehardness of the DLC film increases as the proportion of carbon havingthe diamond structure increases. Therefore, in order to increase thehardness of the DLC film, it is necessary to increase the proportion ofcarbon having the diamond structure and to decrease the amount ofhydrogen compounds in the DLC film.

Here, in a state in which the support portion 21A of the first conductor21 supports the workpiece W in the reactor 12, In a case where theintensity SMW of the microwaves output from the high-frequency outputdevice 30 is gradually increased from “0”, the relationship between theintensity SMW and the bias current, the inventors obtained the followingknowledge.

That is, as shown in FIG. 3, in a stage in which the intensity SMW ofthe microwaves is relatively low, the bias current BA of the workpiece Whardly changes although the intensity SMW of the microwaves increases.However, when the intensity SMW of the microwaves exceeds a firstintensity SMW1, step-up of the bias current BA occurs. Thereafter, whenthe intensity SMW of the microwaves is further increased, the biascurrent BA gradually increases as the intensity SMW of the microwavesincreases. When the intensity SMW of the microwaves exceeds a secondintensity SMW2 which is higher than the first intensity SMW1, step-up ofthe bias current BA occurs again. Thereafter, when the intensity SMW ofthe microwaves is further increased, the bias current BA graduallyincreases as the intensity SMW of the microwave increases. That is, itcan be inferred that the characteristics of materials adhering to theworkpiece W vary between a case where the intensity SMW of themicrowaves is equal to or lower than the first intensity SMW1, a casewhere the intensity SMW is higher than the first intensity SMW1 andlower than the second intensity SMW2, and a case where the intensity SMWis equal to or higher than the second intensity SMW2.

In addition, when the microwaves propagated to the workpiece W andplasma is generated around the workpiece W, a spectrum of the plasma asshown in FIG. 4 can be acquired by a well-known method. In FIG. 4,emission intensities in a wavelength region having wavelengths of “400to 450 nm” are attributable to hydrogen compounds (C_(n)H_(m)) havingcarbon atoms and hydrogen atoms, and a higher emission intensity in thewavelength region can be regarded as a larger amount of hydrogencompounds in the plasma. In addition, both emission intensities in awavelength region having wavelengths of “460 to 480 nm” and emissionintensities in a wavelength region having wavelengths of “510 to 550 nm”are attributable to carbon molecules (C₂) having two carbon atoms, and ahigher emission intensity in each of the wavelength regions can beregarded as a larger amount of carbon molecules in the plasma. In thisspecification, the emission intensities attributable to the hydrogencompounds are referred to as a “C_(n)H_(m) emission intensity”, and theemission intensities attributable to the carbon molecules having twocarbon atoms are referred to as a “C₂ emission intensity”. In thehydrogen compounds “C_(n)H_(m)”, “n” represents the number of carbonatoms and “m” represents the number of hydrogen atoms.

By extracting each of the C₂ emission intensity and the C_(n)H_(m)emission intensity from the spectrum of the plasma, a C₂/C_(n)H_(m)emission intensity ratio which is the ratio of the C₂ emission intensityto the C_(n)H_(m) emission intensity can be calculated. The emissionintensity ratio changes as shown in FIG. 5 when the intensity SMW of themicrowaves is gradually increased from “0”. That is, as shown in FIG. 5,in a stage in which the intensity SMW of the microwaves is relativelylow, the emission intensity ratio is substantially “0” although theintensity SMW of the microwaves increases. That is, the C₂ emissionintensity is substantially “0”. However, when the intensity SMW of themicrowaves exceeds the first intensity SMW1, step-up of the C₂ emissionintensity occurs, and step-up of the emission intensity ratio alsooccurs. Thereafter, when the intensity SMW of the microwaves is furtherincreased, the emission intensity ratio gradually increases as theintensity SMW of the microwaves increases. When the intensity SMW of themicrowaves exceeds the second intensity SMW2, step-down of the emissionintensity ratio C₂ occurs, and thus step-down of the emission intensityratio occurs.

That is, the inventors discovered that according to the intensity SMW ofthe microwaves output from the high-frequency output device 30 (that is,the intensity of the microwaves propagated to the workpiece W), thedecomposition behavior of the acetylene in the reactor 12 is changed(see FIG. 5), and the way that the bias current BA of the workpiece Wchanges is changed (see FIG. 3).

In addition, in this specification, the generation of plasma after theintensity SMW of the microwaves output from the high-frequency outputdevice 30 is set to be equal to or lower than the first intensity SMW1is referred to as “mode 0”. In addition, the generation of plasma afterthe intensity SMW of the microwaves is set to be higher than the firstintensity SMW1 and lower than the second intensity SMW2 is referred toas “mode 1”. Furthermore, the generation of plasma after the intensitySMW of the microwaves is set to be higher than the second intensity SMW2is referred to as “mode 2”.

Here, it is thought that when the acetylene is decomposed into plasma,the acetylene exhibits decomposition behaviors as represented by thefollowing reaction equations (Equation 1), (Equation 2), (Equation 3),and (Equation 4).

C₂H₂→C₂H₂*  (Equation 1)

C₂H₂→C₂H+H  (Equation 1)

C₂H₂→C₂+2H  (Equation 1)

C₂H₂→2C+2H  (Equation 1)

The decomposition behavior represented by the reaction equation(Equation 1) occurs when the π bonds between the carbon atoms are brokenwhile the σ bond between the carbon atoms and the bonds between thecarbon atoms and the hydrogen atoms are maintained. The decompositionbehavior represented by the reaction equation (Equation 2) occurs whenonly one of the bonds between the carbon atoms and the hydrogen atoms isbroken while both the π bonds and the σ bond between the carbon atomsare maintained. The decomposition behavior represented by the reactionequation (Equation 3) occurs when the π bonds between the carbon atomsand the bonds between the carbon atoms and the hydrogen atoms are brokenwhile the σ between the carbon atoms is maintained. The decompositionbehavior represented by the reaction equation (Equation 4) occurs whenall of the π bonds and the σ bond between the carbon atoms and the bondsbetween the carbon atoms and the hydrogen atoms are broken.

Since the π bonds between the carbon atoms are bonds that are moreeasily broken than the other bonds (that is, the σ bond and the bondsbetween the carbon atoms and the hydrogen atoms), the decompositionbehavior represented by the reaction equation (Equation 1) among thefour decomposition behaviors more easily occurs than the otherdecomposition behaviors even though the energy input to the acetylene islow. Since the bonds between the carbon atoms and the hydrogen atoms arebonds that are more easily broken than the σ bond between the carbonatoms, the decomposition behavior represented by the reaction equation(Equation 2) more easily occurs than the decomposition behaviorsrepresented by the reaction equations (Equation 3) and (Equation 4) eventhough the energy input to the acetylene is low. The decompositionbehavior represented by the reaction equation (Equation 3) more easilyoccurs than the decomposition behavior represented by the reactionequation (Equation 4) even though the energy input to the acetylene islow. The decomposition behavior represented by the reaction equation(Equation 4) is less likely to occur unless the energy input to theacetylene is high.

In the mode 0, as is apparent in FIG. 5, the C₂/C_(n)H_(m) emissionintensity ratio is extremely low. It can be inferred that this is due tothe following reason. That is, in the mode 0, the intensity of themicrowaves propagating to the workpiece W is low, and the energy inputto the acetylene in the plasma is low. Therefore, the decompositionbehavior represented by the reaction equation (Equation 1) and thedecomposition behavior represented by the reaction equation (Equation 2)easily occur, while the decomposition behavior represented by thereaction equation (Equation 3) and the decomposition behaviorrepresented by the reaction equation (Equation 4) are less likely tooccur. Moreover, although it is said that the decomposition behaviorrepresented by the reaction equation (Equation 1) and the decompositionbehavior represented by the reaction equation (Equation 2) easily occur,in one acetylene molecule, only the π bonds between the carbon atoms andone of the bonds between the carbon atoms and the hydrogen atoms arebroken. As a result, in the mode 0, carbon molecules (C₂) having twocarbon atoms are rarely generated, and hydrogen compounds (C_(n)H_(m))having carbon atoms and hydrogen atoms are easily generated.Accordingly, in the mode 0, the C₂/C_(n)H_(m) emission intensity ratiois extremely low.

In addition, in the mode 0, since the decomposition behavior representedby the reaction equation (Equation 3) or the reaction equation (Equation4) is less likely to occur, in the DLC film formed on the workpiece W inthe mode 0, the amount of the hydrogen compounds is large, the contentof dangling bonds is low. Furthermore, as described above, since the πbonds between the carbon atoms are less likely to be broken in the mode0, in the DLC film, the proportion of carbon having the diamondstructure among the types of carbon contained in the film is low.

In the mode 1, as is apparent in FIG. 5, the C₂/C_(n)H_(m) emissionintensity ratio is higher than that in the mode 0. It can be inferredthat this is due to the following reason. That is, in the mode 1, sincethe intensity of the microwaves propagating to the workpiece W is higherthan that in the mode 0, the energy input to the acetylene in the plasmais higher than in the case of the mode 0. Therefore, in the mode 1, thedecomposition behavior represented by the reaction equation (Equation 3)mainly occurs among the decomposition behaviors represented by thereaction equations (Equation 1) to (Equation 4). As a result, in themode 1, most of the bonds between the carbon atoms and the hydrogenatoms are broken, and thus the amount of generated hydrogen compounds(C_(n)H_(m)) having carbon atoms and hydrogen atoms is small. Inaddition, while the π bonds between the carbon atoms are broken, a largeamount of σ the bonds between carbon atoms remain, and thus the amountof generated carbon molecules (C₂) having two carbon atoms is large.Accordingly, in the mode 1, the C₂/C_(n)H_(m) emission intensity ratiois high.

In addition, in the mode 1, since the decomposition behavior representedby the reaction equation (Equation 3) mainly occurs, in the DLC filmformed on the workpiece W in the mode 1, the amount of hydrogencompounds is small, and the content of dangling bonds is high.Furthermore, as described above, in the mode 1, while the π bondsbetween the carbon atoms are broken, a large amount of σ bonds betweenthe carbon atoms remain, and the proportion of carbon having the diamondstructure among the types of carbon contained in the film is high in theDLC film.

In the mode 2, as is apparent in FIG. 5, the C₂/C_(n)H_(m) emissionintensity ratio is higher than that in the mode 0. In addition, theC₂/C_(n)H_(m) emission intensity ratio is lower than the maximum valuein the case of the mode 1 (the value of the C₂/C_(n)H_(m) emissionintensity ratio when the intensity SMW of the microwaves is slightlylower than the second intensity SMW2). It can be inferred that this isdue to the following reason. That is, in the mode 2, the intensity ofthe microwaves propagating to the workpiece W is very high, and theenergy input to the acetylene in the plasma is very high. Therefore, thedecomposition behavior represented by the reaction equation (Equation 4)mainly occurs. In this case, a larger amount of bonds between the carbonatoms and the hydrogen atoms are broken, and thus the amount ofgenerated hydrogen compounds (C_(n)H_(m)) having carbon atoms andhydrogen atoms is small. In addition, in the mode 2, a slight amount ofacetylene that exhibits the decomposition behavior represented by thereaction equation (Equation 3) is present, and thus a slight amount ofcarbon molecules (C₂) having two carbon atoms are generated. As aresult, the C₂/C_(n)H_(m) emission intensity ratio in the mode 2 has avalue as shown in FIG. 5.

In the mode 2, since the decomposition behavior represented by thereaction equation (Equation 4) mainly occurs, in the DLC film formed onthe workpiece W in the mode 2, the content rate of dangling bonds in theDLC film is high, and the amount of hydrogen compounds in the DLC filmis small. In addition, as described above, in the mode 2, most of the πbonds and the σ bond between the carbon atoms are broken. When theacetylene is decomposed into plasma as described above, carbon atoms maybe bonded together again in a process of adhering to the workpiece W.However, in order for carbon atoms to be bonded together to form σbonds, higher energy than that in a case where carbon atoms are bondedtogether to form π bonds is necessary. That is, carbon atoms easily formπ bonds but are less likely to form σ bonds. Therefore, in the DLC filmformed on the workpiece W, the number of molecules with carbon atomsforming σ bonds is small, and the proportion of carbon having thediamond structure among the types of carbon contained in the DLC film islow.

Here, in the film forming method of this embodiment, film formation isperformed in the mode 1 in a state in which the workpiece W is chargedwith a negative charge. As a result, a DLC film which satisfies all of asmall amount of hydrogen compounds, a high content of dangling bonds, ahigh proportion of carbon having the diamond structure among the typesof carbon contained in the film is formed on the workpiece W.

Incidentally, both of the first intensity SMW1, which is the intensityof the microwaves as the boundary between the mode 0 and the mode 1, andthe second intensity SMW2 which is the intensity of the microwaves asthe boundary between the mode 1 and the mode 2 are changed according tothe pressure in the reactor 12, the concentration of the acetylene inthe mixed gas supplied into the reactor 12, and the magnitude of the DCvoltage supplied from the DC power source 31.

That is, FIG. 6 shows changes in the first intensity SMW1 and the secondintensity SMW2 when the pressure in the reactor 12 is changed while theconcentration of the acetylene in the mixed gas supplied into thereactor 12 and the magnitude of the DC voltage supplied from the DCpower source 31 are fixed. In FIG. 6, a boundary line L11 represents thefirst intensity SMW1, and a boundary line L12 represents the secondintensity SMW2. As shown in FIG. 6, when the pressure in the reactor 12is gradually increased, both the first intensity SMW1 and the secondintensity SMW2 decrease. However, when the pressure in the reactor 12becomes higher than a certain pressure, both the first intensity SMW1and the second intensity SMW2 increase as the pressure increases fromthe certain pressure.

In addition, FIG. 7 shows changes in the first intensity SMW1 and thesecond intensity SMW2 when the concentration of the acetylene in themixed gas supplied into the reactor 12 is changed while the pressure inthe reactor 12 and the magnitude of the DC voltage supplied from the DCpower source 31 are fixed. In FIG. 7, a boundary line L21 represents thefirst intensity SMW1, and a boundary line L22 represents the secondintensity SMW2. As the concentration of the noble gas in the mixed gasincreases, plasma is easily generated. Therefore, as shown in FIG. 7,both the first intensity SMW1 and the second intensity SMW2 decrease asthe concentration of the acetylene of the mixed gas decreases, that is,as the concentration of the noble gas increases.

In addition, FIG. 8 shows changes in the first intensity SMW1 and thesecond intensity SMW2 when the magnitude of the DC voltage supplied fromthe DC power source 31 is changed while the pressure in the reactor 12and the concentration of the acetylene in the mixed gas supplied intothe reactor 12 are fixed. In FIG. 8, a boundary line L31 represents thefirst intensity SMW1, and a boundary line L32 represents the secondintensity SMW2. As the magnitude of the DC voltage supplied from the DCpower source 31 increases, the amount of the negative charge of theworkpiece W increases. Therefore, as shown in FIG. 8, both the firstintensity SMW1 and the second intensity SMW2 decrease as the DC voltagesupplied from the DC power source 31 increases.

Next, the comparison between the characteristics of the DLC films formedon the workpiece W in the respective modes will be described withreference to FIGS. 9 and 10. In FIG. 9, “●” is a plot of the coefficientof friction of the DLC film, and “⋄” is a plot of the content ofdangling bonds of the DLC film. In FIG. 10, “●” is a plot of thehardness of the DLC film, and “⋄” is a plot of the proportion of carbonhaving the diamond structure in the DLC film.

As shown in FIG. 9, in the DLC film formed on the workpiece W in themode 0, the content of dangling bonds is lower than the contentsdangling bonds of the DLC films formed on the workpiece W in the mode 1and the mode 2. As a result, on the surface of the DLC film formed inthe mode 0, hydroxy groups are less likely to be bonded to carbon atoms,and the coefficient of friction of the DLC film becomes high. Contraryto this, in the DLC films formed in the mode 1 and the mode 2, thecontent of dangling bonds is high, and thus hydroxy groups are easilybonded to carbon atoms on the surface. As a result, the coefficients offriction of the DLC films are low.

In addition, as shown in FIG. 10, in the DLC film formed on theworkpiece W in the mode 1, the proportion of carbon having the diamondstructure is higher than the proportions of the DLC films formed on theworkpiece W in the mode 0 and the mode 2. As a result, the hardness ofthe DLC film formed in the mode 1 is higher than the hardnesses of theDLC films formed in the mode 0 and the mode 2.

More specifically, as microwaves having an intensity SMW of higher thanthe intermediate value (=(SMW1+SMW2)/2) between the first intensity SMW1and the second intensity SMW2 and lower than the second intensity SMW2are output from the high-frequency output device 30, a DLC film whichachieves both high hardness and a low coefficient of friction to a highlevel is formed on the workpiece W. In addition, the two-dot chain linein FIGS. 9 and 10 is a line representing the intermediate value.

Here, as a film forming method of a comparative example, a method usinga PCVD apparatus in which a dielectric is interposed between a workpieceW installed in a reactor and a waveguide will be described. In the filmforming method of this comparative example, plasma is generated aroundthe dielectric in the reactor. Therefore, in order to cause acetylenedecomposed into the plasma to adhere to the entire workpiece W supportedby the dielectric, it is necessary to cause microwaves having a higherintensity to propagate to the dielectric. That is, although filmformation in the mode 2 is possible, film formation in the mode 1 cannotbe performed. Therefore, a DLC film formed by the film forming method ofthis comparative example has a low coefficient of friction but has lowhardness.

Contrary to this, in the film forming method of this embodiment,microwaves can be directly input to the workpiece W, and thus plasma isgenerated around the workpiece W in the reactor 12. Therefore, even whenmicrowaves having a lower intensity than that in the case of the filmforming method of the comparative example are output from thehigh-frequency output device 30, it is possible to form a DLC film overthe entire workpiece W. That is, film formation in the mode 1 becomespossible.

As described above, according to the film forming method of thisembodiment, the following effects can be obtained. (1) In thisembodiment, the DLC film is formed on the workpiece W in the mode 1.Therefore, a DLC film which achieves both high hardness and a lowcoefficient of friction to a high level can be formed on the workpieceW.

(2) Furthermore, when a film is formed on the workpiece W, the workpieceW is charged with a negative charge. Therefore, compared to a case offorming a film on the workpiece W which is not charged with a negativecharge, acetylene decomposed into plasma in the reactor 12 is morelikely to adhere evenly to the entire workpiece W.

(3) Moreover, in this embodiment, the first conductor 21 for supplyingthe microwaves to the workpiece W is also used for supplying DC currentto the workpiece W. Therefore, compared to a case where a supply pathfor DC current is provided separately from a supply path for microwaves,film formation on the workpiece W can be performed with an apparatushaving a simple configuration. In addition, since the second conductor22 is disposed on the outer peripheral side of the first conductor 21,leakage of the microwaves to the outside of the apparatus can beprevented.

The above-described embodiment may be changed to other embodimentsdescribed below. The noble gas supplied into the reactor 12 togetherwith the acetylene may also be a gas other than argon (krypton, xenon,or the like).

-   -   A supply path for DC current to the workpiece W may be provided        separately from the waveguide member 20 as long as the workpiece        W installed in the reactor 12 can be charged with a negative        charge. The workpiece W may not be charged with a negative        charge as long as film formation on the workpiece W in the mode        1 is possible. In this case, as is also apparent in FIG. 8, it        is thought that the first intensity SMW1 and the second        intensity SMW2 are higher than those in the above-described        embodiment.

The first conductor 21 may not have a cylindrical (hollow) shape as inthe above-described embodiment as long as microwaves flow on thesurface, and may have, for example, a columnar (solid) shape. Inaddition, the hydrocarbon gas usable in the film forming method may be agas other than acetylene as long as the following conditions aresatisfied. (Condition 1) A gas having two or more carbon atoms and twoor more hydrogen atoms. (Condition 2) A gas which cause step-up of thebias current of the workpiece W only twice in a process of graduallyincreasing the intensity of the microwaves propagating to the workpieceW from “0”.

What is claimed is:
 1. A plasma chemical vapor deposition apparatus forforming a diamond-like carbon film on a workpiece installed in a reactorby decomposing a hydrocarbon gas supplied into the reactor into plasma,the apparatus comprising: a high-frequency output device which outputsmicrowaves; and a waveguide member which extends to an inside of thereactor from an outside of the reactor, supports the workpiece with aportion of the waveguide member positioned in the reactor, and causesthe microwaves output from the high-frequency output device to propagateto the workpiece, wherein the high-frequency output device outputs themicrowaves having an intensity of higher than a first intensity andlower than a second intensity when a film is formed on the workpiecesupported by the waveguide member, in a process of gradually increasingan intensity of the microwaves propagating to the workpiece through thewaveguide member from “0”, the intensity of the microwaves output fromthe high-frequency output device when step-up of a bias current of theworkpiece occurs is referred to as the first intensity, and in a processof gradually increasing the intensity of the microwaves from the firstintensity, the intensity of the microwaves when step-up of the biascurrent of the workpiece occurs again is referred to as the secondintensity.